Orion is to extend its early reader series after achieving sales of 1.3 million units for the series, which launched with 10 titles in 2009. It will publish 23 titles in 2012, and introduce a guidance system to indicate reading levels including "very easy", "easy" or "confident".
Orion will also develop more new titles for the series. When it launched, the company was looking for ways to repurpose existing stories for younger readers, especially by authors such as Horrid Henry creator Francesca Simon.
Spokeswoman Nina Douglas said: "We are not the first to be publishing early readers but they have had a good impact, partly because of the branding, and because we have used authors and illustrators who children recognise. Now we want to expand beyond this."
The Horrid Henry readers have been its biggest sellers to date, and next year Orion will include six more Horrid Henry tales, as well as other individual works by Simon. The series will also feature stories by authors including Liz Kessler, Vivian French and Marcus Sedgwick.